1. Field of the Invention
The present invention relates to a liquid circulation apparatus, an image forming apparatus and a liquid circulation method, and more particularly, to technology for circulating liquid in the vicinity of a plurality of nozzles of a liquid ejection head which ejects ink droplets from the nozzles.
2. Description of the Related Art
An inkjet recording apparatus has been known which performs recording by ejecting ink droplets toward a recording medium from a plurality of nozzles which are formed in an inkjet head (hereinafter, called a “recording head” or simply “head”). The inkjet recording apparatus has been commonly used because of its little noise during operation, low running cost and capability of recording a high quality image on a wide variety of recording medium. The ink ejection method may be a piezoelectric method in which ink droplets are ejected from nozzles by utilizing the displacement of piezoelectric elements to pressurize the ink inside pressure chambers, or a thermal method in which ink droplets are ejected from nozzles due to the pressure created by the growth of gas bubbles which are generated inside pressure chambers by means of the thermal energy created by heating elements, such as heaters, or the like.
In an inkjet recording method, if an ink having a solvent that is liable to evaporate at the operational temperature and humidity conditions is used (for example, an ink which uses water as a solvent, or the like), then during printing and during standby for printing, a phenomenon occurs whereby the solvent in the ink evaporates from the nozzles, the concentration of solvent in the ink in the vicinity of the nozzles becomes lower, and the viscosity of the ink rises. When the ink viscosity in the vicinity of the nozzles rises, then the fluid resistance inside the nozzles becomes greater, and ejection defects arise due to the occurrence of variations in the volume of the ejected ink droplets or in the direction of flight of the droplets, or ejection failures may occur. Consequently, this may give rise to displacement of the dot positions on the print medium, error in the size of the dots, and omission of dots.
If this situation proceeds further, then it becomes impossible to perform ejection completely, and maintenance known as “nozzle cleaning” becomes necessary.
In response to this, according to experimentation carried out by the present inventors and others, it has been confirmed that in a state where no particular countermeasures are implemented, then in the case of an ink which uses water as a solvent, the solvent starts to evaporate from the nozzles and within five or six seconds, ejection defects arise and extremely serious problems occur even under conditions of normal temperature and normal pressure.
In order to prevent this problem, in a piezoelectric type of head which uses an actuator (piezoelectric element), such as a piezo element, a vibration of a level which does not cause ejection of ink from the nozzles is also applied to the ink in the non-ejecting nozzles (non-operational nozzles) which are not performing ink ejection. The ink in the vicinity of the nozzles is thereby mixed up with the ink inside the pressure chambers, and the fall in the solvent concentration of the ink in the nozzle sections is restricted. Thus, control is implemented which suppresses increase in the viscosity of the ink in the nozzle sections. Below, control of this kind is called “meniscus shaking”.
However, even with this method, if a long period of time elapses, then there is a decline in the concentration of ink solvent in the whole of the pressure chambers and the nozzle sections, and consequently, this can lead to ejection defects. Therefore, before ejection defects arise, the ink in the whole of the pressure chambers needs to be expelled by dummy ejection or by a suctioning operation, and needs to be replaced with fresh ink.
In a head based on a thermal method in which it is difficult to mix up the ink by applying a vibration of a level that does not cause ejection, to the ink, as described above, the ejection force is inherently a strong force, and therefore it takes a long time until ejection defects such as those described above arise. However if left without taking any particular countermeasures, ejection defects will arise, and therefore control is implemented in a similar fashion in order to expel ink which has risen in viscosity in the vicinity of the nozzles.
Furthermore, a method has also been adopted in a thermal type of head, whereby ink is circulated through a common flow channel, the volume of the pressure chambers is made as small as possible, and the distance between the common flow channel and the nozzles is shortened, in such a manner that decline in the solvent concentration of the ink in the vicinity of the nozzles is delayed by the effects of the diffusion of solvent from the common flow channel. However, in order completely to prevent increase in the viscosity of the ink in the vicinity of the nozzles by means of this method, it is necessary for the length from the supply path to the nozzle, via the pressure chamber, to be no more than several tens of microns, and therefore, in practice, the increase in viscosity is not suppressed completely, and control for expelling ink of raised viscosity is still needed.
In inkjet printers using either of the aforementioned kinds of actuator, printing cannot be carried out while ink is being expelled, and therefore, the ink of raised viscosity is expelled by moving the head to a position that is distanced from the printing region, or alternatively, if the print medium is a cut paper, or the like, ink is expelled by providing a medium for receiving the ink expelled in the interval between respective sheets of the print medium.
In other words, it is not possible to prevent increase in the viscosity of the ink due to evaporation of the solvent, either by shaking the meniscus, or by employing a diffusion effect by reducing the volume of the pressure chambers, or the like, and therefore ink of raised viscosity is required to be expelled and discarded, giving rise to wasted ink.
Even if this ink is reused rather than being discarded, a filtering process is required since there is a high probability that dust, or the like, will have entered into the ink having been expelled from the nozzles.
Since it is essentially impossible to carry out printing during the expulsion of ink, whichever of the printing systems described above is adopted, then there is a problem in that productivity declines.
In response to problems of this kind, technology has been proposed for preventing decline in the concentration of ink solvent in the vicinity of nozzles by constantly circulating the ink in non-ejecting nozzles and ejecting nozzles, during printing (see, for example, Japanese Patent Application Publication No. 63-41152, Japanese Patent Application Publication No. 1-108056, Publication of Japanese translation of PCT Application No. 2000-512233 and Publication of Japanese translation of PCT Application No. 2003-505281.)
However, there are problems of the following kinds associated with these ink circulation technologies in the related art.
(1) If it is sought to maintain a good printing state for all of the printing conditions, then this implies the most severe conditions in terms of the volume of circulated ink, and therefore the total volume of reused ink after circulation becomes very large, and the amount of added solvent also becomes large.
(2) If a filter is used to deal with any possible infiltration of foreign matter, then the lifespan of the filter is very short.
(3) In the case of UV-curable ink, if circulation is continued then the ink becomes less readily curable, due to the effects of the oxygen and moisture in the air (in the case of a radical type of UV-curable ink, the presence of oxygen causes the radicals to be captured by the oxygen, thereby inhibiting the curing reaction, and in the case of a cationically-curable ink, the presence of moisture makes curing difficult to achieve). Furthermore, the ink is degraded and may become unrecoverable (irreversibly changed), due to chemical changes caused by the effects of heating due to the temperature adjustment of the head, or light of trace levels, or the like.
(4) Since air becomes dissolved in the ink that makes contact with the air in the nozzle sections, then if the circulation volume is high, the amount of dissolved air in the ink increases, the compliance of the ink changes, and the ejection characteristics hence change. Moreover, the time taken for the air bubbles to disappear also becomes longer in the event that air bubbles do enter into the ink, and the restoration time for the effects caused by the air bubbles becomes longer.
Consequently, it is desirable for the amount of circulated ink to be as small as possible.